This invention relates to a timer circuit for DC motor control, and more particularly to a pre-step timer circuit for operating a DC crane-hoist control system.
Typically, a timer module in a DC crane-hoist control system is a two-terminal device. The purpose of the timer module is to provide a fixed time delay between speed points or control steps. This allows time for the motor to accelerate and to develop sufficient speed, which, in turn, limits the motor current inrush to an acceptable maximum value. The timer module is wired in series with an accelerating contactor coil. It appears as a high impedance during its timing period. The timing period of a timer module is initiated by application of a voltage thereacross when a master switch or the like is advanced to that control step. With the contacts of the master switch closed for that control step, a trickle current flows through the timer module which is insufficient to pick-up the accelerating contactor coil in series therewith. Therefore, when a master switch of a DC motor controller is moved from a previous control step to the next control step, the energization of the accelerating contactor coil in the next control step is delayed by the timer module for a predetermined fixed time, called an anticipation delay. Upon the completion of the timing cycle, the timer module appears as a contact closure (low series impedance ) and, therefore, a greater flow of current occurs whereby the accelerating contactor coil is energized. The timing cycle of the timer module is obtained by use of a resistor-capacitor exponential curve and a programmable unijunction transistor voltage comparator (hereinafter called P.U.T.). Upon completion of the required timing period, or anticipation delay period, the P.U.T. provides a current pulse to the gate of a silicon controlled rectifier (hereinafter called SCR). The SCR then goes into conduction and provides a low impedance path or, in other words, a contact closure.
Next, when the master switch is advanced to the next control step, the accelerating contactor of the next control step is delayed by the timer module for its predetermined fixed time. This anticipation delay is a disadvantage when the operator is using the crane-hoist control in a "jogging" sequence. A jogging sequence is when the master switch of the controller is maintained in a control step (speed point), such as the first speed point, and momentarily moved to the next speed point for positioning of a load, etc. When only the timer module circuits of prior art crane-hoist controllers are used, the operator must anticipate this time delay, since the timer module of the next higher speed is not initiated until the master switch is advanced to that next higher speed position. It was in an effort to provide a crane-hoist control system for DC motors, which can be run in a jogging sequence without the anticipation time delay between control steps or speed points on the master switch of the DC motor controller, that the present invention came about.